linux-kernel - Re: [Linux Kernel 5.13 GA] ESXi Performance regression

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Message-ID: <BYAPR05MB520669C6C1A8A522B1DE0EAAA4F29@BYAPR05MB5206.namprd05.prod.outlook.com>
Date:   Thu, 5 Aug 2021 14:33:52 +0000
From:   Rahul Gopakumar <gopakumarr@...are.com>
To:     Valentin Schneider <valentin.schneider@....com>,
        Abdul Anshad Azeez <aazees@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Rajender M <manir@...are.com>
Subject: Re: [Linux Kernel 5.13 GA] ESXi Performance regression

> sysctl_sched_wakeup_granularity's default value hasn't been touched since
> 2009:
> 
> 172e082a9111 ("sched: Re-tune the scheduler latency defaults to decrease worst-case latencies")
>
> and the automagic scaling (see kernel/sched/fair.c::update_sysctl()) hasn't
> changed much either.
> 
> What's likely to happen here is that you have a service in your distro (or
> somesuch) tweaking those values and since the incriminated commit moves
> those files to /sys/kernel/debug/sched/, said service doesn't do anything
> anymore.

Hi Valentin,

In our testing, we use RHEL 8.1 distro. It looks like tuned daemon updates
15ms (per tuned virtual-guest profile) in /proc/sys/kernel/sched_wakeup_granularity_ns
file during tuned's startup.

Now tuned daemon is not able to update the value as the commit moves those
files to debugs and thus sched_wakeup_granularity_ns file remains with the
default value.

Regards,
Rahul Gopakumar,
Performance Engineering,
VMware, Inc.